COMSPHERE 3600 Series Data Service Units
5-20 March 1999 3610-A2-GB41-60
Digital Loopback
Digital Loopback (DL) is used for manual testing of
the remote end of the circuit. For example, a remote
Digital Loopback may be required in order to complete an
external Bit Error Rate Test (BERT) from the remote
DSU. The local DSU receives test data, loops it back to
the transmitter before the DTE interface, and returns it to
the network (Figure 5-7).
For the DSU, the Digital Loopback command is
session-disruptive except when a dial backup session is
active. It is used to condition the DSU for testing to
determine whether the DSU connection to the network is
functioning properly. This test requires a DDS network
circuit between two DSUs. DTE connections to the DSUs
are not required.
For the DBM, Digital Loopback is used to determine
whether the DBM connection to the dial network is
functioning properly. Digital Loopback is session-
disruptive when the DBM is in Dial Backup mode and is
nondisruptive when it is in Standby mode. The test
requires a dialed connection to another DBM. DTE
connections to the DBMs are not required.
Digital Loopback (DL) can be initiated by:
•Selecting DL from the Lpbk submenu in the Test
branch menu.
•Receiving a V.54 loopback pattern. Enable
configuration options V.54 Lpbk (V.54 Loopback)
and RespondRDL (Respond to Remote Digital
Loopback).
A Digital Loopback may not be initiated from a remote
multipoint device.
NOTE
If the Bilat Lpbk (Bilateral
Loopback) configuration option is
enabled, requesting a Digital
Loopback automatically initiates a
DTE Loopback as well.
Figure 5-7. Digital Loopback